1. Field of the Invention
The present invention relates to a constant voltage/frequency power supply apparatus capable of continuously applying a constant voltage having a constant frequency to a load.
2. Description of the Prior Art
In general, a constant voltage/frequency power supply apparatus is used to apply a constant voltage having a constant frequency to loads such as devices and appliances that are required to be continuously driven in a constant condition.
In FIG. 1, there is shown a block diagram of one conventional constant voltage/frequency power supply apparatus. Reference numeral 1 indicates an AC (alternating current) power supply, reference numeral 2 denotes a transformer for transforming an output voltage of the AC power supply 1, reference numeral 3 represents a constant voltage/frequency output circuit (referred to as "a CVCF circuit") to obtain a constant voltage having a constant frequency by processing the output voltage of the transformer 2, and reference numeral 4 denotes another transformer for transforming the constant voltage/frequency output derived from the CVCF circuit 3 so as to supply the transformed constant voltage/frequency output to a load 5. The above-described CVCF circuit 3 includes a converter 31 for converting the AC output supplied from the AC power supply 1 via the transformer 2 into a DC (direct current) output, an inverter 32 for inverting the DC output of the converter 31 into a corresponding AC output, and an AC filter 33 for filtering the AC output inverted by the inverter 32. The AC inverter 32 is connected to a control circuit 6.
The above-mentioned CVCF circuit 3 is illustrated in FIG. 2, more particularly. In FIG. 2, the converter 31 is composed of a bridge circuit made up of four diodes 311-314 and an electrolytic capacitor 315. The inverter 32 consists of four transistors 321-324, each connected with the control circuit 6 which outputs the signal controlling the inverter 32 for forming a square wave including output frequency components between points A and B by turning transistors on or off. The AC filter 33 is made up of a reactor 331 and a capacitor 332.
With this circuit arrangement, the AC output derived from the AC power supply 1 is transformed to a predetermined AC voltage by the transformer 2, and then this AC voltage is input to the CVCF circuit 3. Upon receipt of this AC voltage in the CVCF circuit 3, it is converted into the DC voltage by the converter 31, the DC output is inverted into the AC output by the inverter 32 which is conducting-controlled by control signals of the control circuit 6. Thereafter, the inverted AC output is filtered by the AC filter 33 to derive only a desirable frequency component therefrom, which is then transformed by the transformer 4 to produce a desired AC voltage. The resultant AC voltage is a constant AC voltage having a constant frequency, which is to be applied to the load 5.
Another prior art constant voltage/frequency power supply apparatus is shown in FIG. 3, the circuit arrangement as well as the operation of which are different from those of the power supply apparatus shown in FIG. 1. It should be noted that reference numerals 1 through 6 and 31 to 33 shown in FIG. 3 denote the same circuit elements shown in FIG. 1. In the circuit of FIG. 3, a changeover circuit 7 is interposed between the CVCF circuit 3 and the transformer 4, and a direct voltage-applying circuit 8 is connected to a junction between the transformer 2 and the CVCF circuit 3, and the changeover circuit 7. The changeover circuit 7 is composed of a contactor, a thyristor, or a transistor.
A description will now be made of the operation of the circuit arrangement shown in FIG. 3. As the operation of the CVCF circuit 3 including the control circuit 6 is the same as that of FIG. 1, the constant AC voltage having a constant frequency can be applied to the load 5. When such a constant AC voltage having a constant frequency is not required for the load 5, or when an accident occurs in the CVCF circuit 3 in case of providing such a constant AC voltage having a constant frequency, the AC output transformed by the transformer 2 may be supplied directly to the load 5 via the direct voltage-applying circuit 8 by changing the changeover circuit 7 such as AC switches, as illustrated in FIG. 3.
The above-described conventional constant voltage/frequency power supply apparatus, however, contain the following drawbacks.
First, since the output voltage of the CVCF circuit 3 appears as a sine wave, a so-called "drift" phenomenon occurs due to changes in the control element characteristics by the aging and the temperature variations, and commands on the reference sine wave are changed, then DC magnetic polarization is produced by superimposing a DC component on the AC output. If the DC magnetic polarization occurs in the power supply apparatus, the transformer 4 connected to the output stage of the CVCF circuit 3 is excited by this DC component. As a result, the DC current flows through the primary coil of the transformer 4, resulting in an increase of the temperature, which causes the transformer to be burned out.
Secondly, if the circuit elements of the inverter 32 in the CVCF circuit 3 are broken down due to certain reasons, the polarity of the DC voltage accidentally appears as a single polarity. The conventional power supply apparatus, however, cannot detect such a breakdown accident instantaneously.